A language spec (other than "cpython is the spec") would probably be good. But I know of jython, iron python, stackless python and micro python, in addition to pypy -- that are all more or less complete and/or alive as alternative implementations. I'm not sure of the .net variant (iron python) -- I've heard rumours that it's dead. So it's not like the lack of a spec is an insurmountable obstacle, nor like it's not something that no-one is working on...
It's probably not an insurmountable obstacle, but getting over it is by no means trivial. Exhibit A: None of the alternative Python implementations are 100% compatible with CPython. I get the impression that, in practice, the "Python spec" is something that can only be approached asymptotically.
